1. Field of the Invention
This invention relates to a method for forming a superconductor thin film and an apparatus therefor, more particularly to a method for forming a thin film of a superconductor comprising a composite oxide containing an element such as a rare earth element, Bi or Tl, and an ultrasonic wave sprayer used therefor.
2. Descriptions of Related Art
Recently, many superconductive materials having a high critical temperature have been developed. For example, Dr. J. G. Bednorz et al of the Zurich Research of IBM reported that a barium-lanthanum-copper-oxygen composite metal oxide exhibited superconductivity at a relatively high temperature, 30 K (see Z. Phys. B-Condensed Matter, Vol. 64, 1986, pp. 189-193). Then, Dr. Tanaka et al of the Tokyo University reported that composite metal oxides represented by the formulae: (La.sub.l-x Sr.sub.x).sub.2 CuO.sub.4-.delta. and (La.sub.l-x Ca.sub.x).sub.2 CuO.sub.4-.delta. exhibited superconductivity at maximum temperature of 37 K and 18 K, respectively (see Chemistry Letters, 1987, pp. 429-432).
Furthermore, Dr. C. W. Chu et al of Houston University reported that they found a barium-yttrium-copper-oxygen composite metal oxide having a critical temperature of 93 K (see Phys. Rev. Letters, Vol. 58, 1987, pp. 908-910). Dr. Kitazawa et al of the Tokyo University reported a barium-ytterbium-copper oxide having a critical temperature of 95 K (see Jap. Journal of Appl. Phys., Vol. 26, Apr., 1987, pp. L339-L341), and Dr. Takagi et al reported that a barium-erbium-copper oxide had a critical temperature of 94 K (see Jap. Journal of Appl. Phys., Vol. 26, May, 1987, pp. L601-L602).
Instead of the rare earth elements above, it is expected that other rare earth element such as Lu, Tm, Ho, Dy, Cd, Nd, Sm or the like will be able to be introduced to obtain superconductors having a higher critical temperature.
Particularly, since a bismuth-strontium-calcium-oxygen superconductor is characterized by not only having a high zero-resistance temperature but also being stable to water, it is highly expectable for various applications. That is, this superconductor is said to be most practically used as electronic elements such as SQUID in which it is in a thin film form.
Also, a thallium-barium-calcium-copper-oxygen superconductor has a highest zero-resistance temperature among prior superconductors and, therefore, it is highly expectable as well as the bismuth-strontium-calcium-oxygen superconductor, although a thallium compound as a starting material is toxic.
These composite metal oxide superconductors are generally prepared by mixing and firing oxides, carbonates or the like of various metals constituting the superconductors.
For the formation of a superconductor thin film, evaporation or sputtering under vacuum has been tried directly on a substrate in a plate form. Introduction of a substrate in a wire form into a bath containing a molten superconductor has been tried to produce the superconductor thin film on the wire-like substrate.
However, the evaporation or sputtering under vacuum has such a drawback that it requires too high a degree of vacuum to produce the superconductor thin film with a high efficiency of production and further in a big size.
The introduction of the wire-like substrate into the bath containing a molten superconductor has such a drawback that the thin film obtained is uneven in thickness.